In a commercial alkylation plant, acid is used as catalyst to produce gasoline by the alkylation of C.sub.3 -C.sub.5 olefins and alkanes. The catalytic acids include sulfuric acid, hydrogen fluoride, and solid acid. Only sulfuric acid and hydrogen fluoride are commercialized nowadays. Because hydrogen fluoride is very toxic, hydrogen fluoride is gradually being phased out in the alkylation process. The most popular catalyst used to manufacture gasoline from the alkylation of C.sub.3 -C.sub.5 olefins and alkanes is concentrated sulfuric acid. The ratio of sulfuric acid to organic compounds in the alkylation reaction vessel is 0.1.about.0.3, so the amount of concentrated sulfuric acid required in the alkylation process is very large. In general, producing one ton gasoline makes 0.1 ton spent sulfuric acid. If ten thousands barrels of gasoline are produced per day, one hundred tons of spent sulfuric acid are produced. Such a large amount of spent sulfuric acid can not be cast away or stored, it must be treated in advance. Based on the above description, the key point of alkylation is the treatment or regeneration of the spent sulfuric acid.
A lot of products during alkylation processes using sulfuric acid as catalyst were raised by L. F. Albright and A. R. Goldsby, "Industrial and Laboratory alkylations" in ACS symposium series 55, p. 91, Washington, D.C., 1977, as shown in Table 1.
TABLE 1 ______________________________________ Composition of alkylates obtained over H.sub.2 SO.sub.4 hydrocarbons composition ratio ______________________________________ isopentane -- n-pentane -- C.sub.5 8.9 2,3-dimethylbutane 4.7 2-methylpentane 1.1 3-methylpentane 0.4 C.sub.6 6.2 2,2,3-trimethylbutane 0.2 2,2-dimethylpentane 0.2 2,4-dimethylpentane total 2-methylhexane 3.4 3-methylhexane 0.3 2,3-dimethylpentane 2.3 C.sub.7 6.4 2,2,4-trimethylpentane 24.3 2,2,3-trimethylpentane 1.2 2,3,3-trimethylpentane 12.3 2,3,4-trimethylpentane 13.0 2,2-dimethylhexane 0.2 2,3-dimethylhexane total 2,4-dimethylhexane 3.0 2,5-dimethylhexane 6.6 3,4-dimethylhexane 0.4 C.sub.8 61.0 C.sub.9 and higher 17.5 ______________________________________
The organic products may reside in the sulfuric acid to form said organic impurities, and the water contained in the raw material of alkylation or generated during alkylation may accumulate in sulfuric acid, so the spent sulfuric acid must be regenerated to remove both the organic impurities and the water.
In order to treat the spent sulfuric acid in a commercial alkylation plant, a sulfuric acid plant is usually set up. The procedure of the treatment of sulfuric acid is described as following. First of all, spent sulfuric acid, air, and fuel are sent into the combustion chamber to burn out the organic impurities and sulfuric acid into SO.sub.2, SO.sub.3, CO.sub.2, H.sub.2 O and ashes, etc. After drying the gases from the combustion chamber, the dried gases are purified to remove impurities and ashes in order to get pure SO.sub.2. The SO.sub.2 gas reacts with air or oxygen to convert to SO.sub.3 at high temperature using V.sub.2 O.sub.5, etc. as catalyst. The SO.sub.3 gas is absorbed twice by water to get concentrated sulfuric acid. The operation of producing sulfuric acid in commercial process is very difficult and the costs of both equipment and operation are very expensive, because the complexity of the process, the corrosion of equipment at high temperature, and the presence of different impurities in the spent catalyst.
From the above description, there are some disadvantages using the traditionally commercialized process to treat the spent sulfuric acid: (1) To recover the regenerated sulfuric acid, several stages including in this process; combustion, purification, oxidation, purification again, and absorption have to be carried out on the spent sulfuric acid. The whole process is very complicated. (2) The corrosive compound is treated at very high temperature, so special material has to be chosen to construct the reactor, etc. (3)Based on the descriptions of (1) and (2), the costs of both equipment-and operation are very expensive obviously. (4) Very large amount of waste water, waste gas, and ashes are produced during the recovery process. Additional investment has to be funded to the facilities of retreatment for the waste materials. The additional investment is substantial. So, the traditionally commercialized process for the recovery of sulfuric acid from the spent catalyst of alkylation is very complicated and expensive.